Process of strengthening dilute phosphoric acid

ABSTRACT

A process for the concentration of dilute phosphoric acid in a phosphoric acid process characterized by a plurality of separate phosphoric acid cycles when the phosphoric acid is concentrated in each cycle by a vacuum evaporation is described wherein the phosphoric acid cycles is heated by indirect heat exchange by heat of formation formed in a sulfuric acid contact process.

This is a continuation of application Ser. No. 41,468, filed May 22,1979 now U.S. Pat. No. 4,276,116, dated June 30, 1981.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a process of concentrating dilute phosphoricacid, which is conducted in a plurality of separate phosphoric acidcycles, which are associated with respective vacuum evaporator stagesconnected in series, and is indirectly heated in each cycle outside theevaporators by means of surplus heat from a contact process sulfuricacid plant.

2. Discussion of the Prior Art

The treatment of raw phosphates with sulfuric acid often results in adilute phosphoric acid which contains about 26 to 32% P₂ O₅ which foruse in the production of fertilizer, must be concentrated to aconcentration of more than about 45% P₂ O₅.

From A.V. Slack "Phosphoric Acid", Vol. II, 1968, published by MarcelDekker, Inc., New York, pages 607 to 634, it is known to concentratephosphoric acid in a process in which the phosphoric acid is conductedin a plurality of separate phosphoric acid cycles, which are associatedwith respective vacuum evaporator stages connected in series, and isindirectly heated in each cycle outside the evaporators by means oflow-pressure steam, which is produced by means of surplus heat from asulfuric acid process contact process. Because much energy is consumedin concentrating phosphoric acid, a large proportion of the high-gradeenergy produced in the form of steam in the sulfuric acid contactprocess plant is spent for such concentrating. Unless it was required inthe form of valuable steam for concentrating the phosphoric acid, theenergy supplied from the sulfuric acid contact process plant could beused to produce more-electric power to meet the total energy requirementof the combined plant for producing sulfuric acid and phosphoric acid.

It is an object of the invention to provide a process for concentratingphosphoric acid in a combined plant for producing sulfuric acid andphosphoric acid which avoids, as far as possible, the use of high-gradeenergy for concentrating the dilute phosphoric acid and provides asubstitute for such energy in a manner which is technologically simpleand economical.

SUMMARY OF THE INVENTION

In accordance with the invention, this object is accomplished in thatthe phosphoric acid in the phosphoric acid cycles is indirectly heatedby means of surplus heat which becomes available in the sulfuric acid inthe SO₃ absorption systems of the contact process sulfuric acid plantand is to be dissipated.

The SO₃ formed in the contact process sulfuric acid plant is absorbed byconcentrated sulfuric acid, which is circulated. In most cases, theabsorption is effected in an interstage absorber and an end absorber.Owing to the heat generated by the exothermic reaction and the sensitiveheat taken up from the gas phase, the acids leaving the absorbers are attemperatures of about 80° to 100° C. and where hot absorption systemsare employed at temperatures of about 200° C. To dissipate the heatwhich has been generated, the sulfuric acid must be cooled as it iscirculated. As a result, thermal energy becomes available at relativelylow temperatures and is usually not utilized. Dilute phosphoric acid isstrengthened in two or more vacuum evaporator stages. In each stage, thephosphoric acid is circulated and phosphoric acid is supplied from eachstage to the next only at the P₂ O₅ rate of the phosphoric acid whichenters the stage. In the phosphoric acid cycle associated with eachevaporator stage, the phosphoric acid is passed through at least oneheat exchanger and is heated therein by means of the heat which is to bedissipated from the sulfuric acid in the absorption systems. Part of thesolids which have settled in the last evaporator stage may be withdrawnand recycled to the phosphoric acid cycle associated with the firststage so that deposition problems, particularly in the heat exchangers,will be avoided. In the vacuum evaporator stages, the phosphoric acid isconcentrated in that water is evaporated in dependence on the watervapor partial pressure under the prevailing conditions of temperature,pressure and phosphoric acid concentration. In addition to water, mostof the fluorine-containing compounds are driven off whereas thephosphoric acid vapor partial pressure is very low.

In one embodiment, the phosphoric acid in the phosphoric acid cycles isheated by an indirect heat exchange between the phosphoric acid and thesulfuric acid in the sulfuric acid cycles of the SO₃ absorption systems.

In that case, the sulfuric acid withdrawn from the absorpting systems isfed direct to the heat exchangers and in the latter indirectly transferits heat to the phosphoric acid. That embodiment will be used mainly innew plants and where the conduits from the absorption systems to theheat exchangers of the phosphoric acid cycles are relatively short.

In another embodiment, the phosphoric acid in the phosphoric acid cyclesis heated by a heat-carrying liquid medium, which is conducted in aclosed cycle and heated by an indirect heat exchange with the sulfuricacid of the sulfuric acid cycles of the SO₃ absorption system. In thatcase the sulfuric acid withdrawn from the absorption systems is fed toheat exchangers which are disposed near the absorption systems and inwhich the heat-carrying medium consisting preferably of water underpressure is heated. Said heat-carrying medium is subsequently suppliedto the heat exchangers of the phosphoric acid cycles. One closed cyclefor the heat-carrying medium or a plurality of such cycles may be used.This embodiment will be used mainly where existing plants comprisingheat exchangers in the absorption systems and in the phosphoric acidcycles are to be altered and where the conduits between the absorptionsystems and the heat exchangers of the phosphoric acid cycles arerelatively long. The large total floor space required for the heatexchangers can be compensated to a large extent by the use of lessexpensive materials.

In a preferred embodiment, the phosphoric acid in the phosphoric acidcycle associated with the first vacuum evaporator stage is heated in twoheat exchangers, which are connected in parallel and one of which issupplied with sulfuric acid from the end absorber, whereas the otherheat exchanger is supplied with part of the sulfuric acid from theinterstage absorber and the phosphoric acid in the phosphoric acid cycleassociated with the second vacuum evaporator stage is heated by theremaining sulfuric acid from the interstage absorber. Each heatexchanger may comprise two or more units connected in parallel. In suchan arrangement, the end absorber of the contact process sulfuric acidplant can be operated so that the acid withdrawn from it is at a lowertemperature than the acid withdrawn from the interstage absorber and thedistribution of heat to the various evaporator stages to the plant forconcentrating phosphoric acid can be well controlled.

BRIEF DESCRIPTION OF DRAWINGS

The invention will be explained more in detail with reference to thedrawings and to Examples.

FIG. 1 is a flow scheme showing a contact process sulfuric acid plantand a plant for concentrating phosphoric acid in an arrangement in whichsulfuric acid from the absorption systems is supplied direct to the heatexchangers of the phosphoric acid cycles.

FIG. 2 is a flow scheme showing a contact process sulfuric acid plantand a plant for strengthening phosphoric acid in an arrangement in whichsulfuric acid from the absorption systems heats a heat-carrying medium,which is then supplied to the heat exchangers of the phosphoric acidcycles.

DESCRIPTION OF SPECIFIC EMBODIMENTS

In accordance with FIG. 1, dilute phosphoric acid is fed in conduit 1 toan evaporator 2 of a first vacuum evaporator stage. By means of a pump4, phosphoric acid is withdrawn in conduit 3 from the evaporator 2 andfed in conduits 5, 6 to heat exchangers 7, 8 and is heated therein andthen returned to the evaporator 2. Phosphoric acid at a P₂ O₅ rate whichis equal to the rate of P₂ O₅ supplied in conduit 1 is fed by means of apump 11 in conduits 10 and 12 to an evaporator 13 of a second vacuumevaporator stage. By means of a pump 15, phosphoric acid is withdrawnfrom the evaporator 13 in conduit 14 and is fed in conduit 16 to a heatexchanger 17 and is heated therein and then returned in conduit 18 tothe evaporator 13. Concentrated phosphoric acid is conducted in conduit19 to a settling tank 20. Product is withdrawn in conduit 21. Part ofthe solids-containing suspension which has settled is fed via conduit22, screw pump 23 and conduit 24 to the evaporator 2 of the first vacuumevaporation stage. The discharge conduits 10 and 19 connected to theevaporators 2 and 13 are vented by vents 25a, 25b.

Hot sulfuric acid is supplied from an end absorber 26 via conduit 27,pump 28, and conduit 29 to the heat exchanger 7 and is returned inconduit 30 to the end absorber 26. Hot sulfuric acid is supplied from aninterstage absorber 31 via conduit 32, pump 33 and conduits 34 and 35 tothe heat exchanger 8 and is returned in conduits 36 and 37 to theinterstage absorber 31. Part of the hot acid withdrawn from theinterstage absorber 31 is fed in conduit 38 to the heat exchanger 17 andis returned in conduits 39 and 37 to the interstage absorber 31.

In accordance with FIG. 2, phosphoric acid flows are as in FIG. 1 butthere is only one heat exchanger 7 in the first vacuum evaporator stage.

Hot sufluric acid is supplied from the end absorber 26 via conduit 27,pump 28 and conduit 40 to a heat exchanger 41 and is returned from thelatter in conduit 42 to the end absorber. A heat-carrying liquid mediumis heated in the heat exchanger 41 and is then supplied in conduit 43 toan intermediate container 44. Hot sulfuric acid is supplied from theinterstage absorber 31 via conduit 45, pump 46 and conduit 47 to a heatexchanger 48 and is returned from the latter in conduit 49 to theinterstage absorber 31. The heat-carrying liquid medium which has beenheated in the heat exchanger 48 is supplied in conduit 50 to theintermediate container 44. A partial stream of the heat-carrying liquidmedium is supplied from the intermediate container 44 via conduit 51,pump 52 and conduits 53 and 53 to the heat exchanger 7 and from thelatter in conduit 55 to an intermediate container 56. The remainingstream of heat-carrying medium is supplied in conduit 58 to theintermediate container 56. A partial stream of the heat-carrying mediumis returned from the intermediate container 56 via conduit 59, pump 60and conduits 61 and 62 to the heat exchanger 41. The remaining partialstream of the heat-carrying medium from the intermediate container 56 isreturned in conduit 63 to the heat exchanger 48.

In all cases, an additional heat exchanger for cooling or heating may beconnected in the circuit for the sulfuric acid or the heat-carryingmedium so that an overall heat balance can be maintained particularlywhen the combined plant is being started or shut down.

In order to more fully illustrate the invention, and the manner ofpracticing the same, the following examples are presented:

EXAMPLES

The phosphoric acid plant is combined with a plant for producingsulfuric acid at a rate of 1500 (metric) tons H₂ SO₄ (100%) per day.

    ______________________________________                                        REFER-                                                                        ENCE                                                                          NUMBER  PARAMETER     UNIT       FIG. 1                                                                              FIG. 2                                 ______________________________________                                        1       Rate of fresh t/h        81.1  81.1                                           phosphoric acid                                                               P.sub.2 O.sub.5 concentration                                                               % by weight                                                                              28.0  28.0                                   2       Vacuum        mbars      240   240                                    3       P.sub.2 O.sub.5 concentration                                                               % by weight                                                                              42    42                                             Solids content                                                                              % by weight                                                                              4                                                    Acid rate     t/h        8300  8300                                           Acid temperature                                                                            °C. 83    83                                     9       Acid temperature                                                                            °C. 86    86                                     12      Phosphoric acid rate                                                                        t/h        54    54                                     13      Vacuum        mbars      70    70                                     14      P.sub.2 O.sub.5 concentration                                                               % by weight                                                                              54    54                                             Acid rate     t/h        4300  4300                                           Acid temperature                                                                            °C. 83    83                                     18      Acid temperature                                                                            °C. 86    86                                     21      Product discharge                                                                           t/h        42    42                                             rate                                                                          P.sub.2 O.sub.5 concentration                                                               % by weight                                                                              54    54                                     22      Solids rate   t/h        3     3                                      27      Acid rate from                                                                              t/h        900   900                                            absorber                                                                      H.sub.2 SO.sub.4                                                              concentration % by weight                                                                              98.5  98.5                                           Acid temperature                                                                            °C. 140   140                                    30, 42  Acid temperature                                                                            °C. 120   120                                    32, 45  Acid rate     t/h        2400  2400                                           H.sub.2 SO.sub.4                                                              concentration % by weight                                                                              98.5  98.5                                           Acid temperature                                                                            °C. 140   140                                    37, 49  Acid temperature                                                                            °C. 120   120                                    35      Acid rate     t/h        1050  --                                     38      Acid rate     t/h        2250  --                                     51      Pressure water rate                                                                         t/h        --    1235                                           Pressure water                                                                              °C. --    120                                            temperature                                                           54      Pressure water rate                                                                         t/h        --    845                                    57      Pressure water rate                                                                         t/h        --    390                                    59      Pressure water                                                                              °C. --    100                                            temperature                                                           ______________________________________                                    

The main advantages afforded by the invention reside in that the energyrequired for concentrating, except for the energy required to generatethe vacuum, can be recovered entirely from low-grade heat from thecontact process sulfuric acid plant. At the same time it is no longernecessary to cool the sulfuric acid cycles of the absorption systems bymeans of cooling water or air so that the resulting thermal loading ofthe environment is avoided. Whereas low-grade thermal energy is used toa large extent, the expediture in equipment and the operating costs arenot higher than were steam, e.g., is used as a heating medium. Asregards the mutual dependence of the phosphoric acid-strengthening plantand the contact process sulfuric acid plant in operation, the processaccording to the invention does not suffer from the process in whichsteam generated in the contact process sulfuric acid plant is used as aheating medium.

    ______________________________________                                        TRANSLATION OF THE                                                            INSCRIPTIONS ON THE DRAWINGS                                                  ______________________________________                                        FIGS. 1 and 2                                                                 Schwefel         Sulfur                                                       Abhitzekessel    Waste heat boiler                                            Uberhitzer       Superheater                                                  Verdampfer       Evaporator                                                   Eco              Economizer                                                   Kontakt          Contact process reactor                                      Zwischen-WA      Interstage heat exchanger                                    Hauptgeblase     Main blower                                                  Luft             Air                                                          Kamin            Chimney                                                      Kuhlwasser       Cooling water                                                H.sub.2 SO.sub.4 -Produktion                                                                   H.sub.2 SO.sub.4 product                                     Trockner         Dryer                                                        Zwischenabsorber Interstage absorber                                          Endabsorber      End absorber                                                 Dampf            Steam                                                        Kuhlwasser       Cooling water                                                Abwasser         Sewage                                                       FIG. 1                                                                        Vakuumsystem     Vacuum system                                                FIG. 2                                                                        Vakuumerzeugung  Vacuum generation                                            ______________________________________                                    

What is claimed is:
 1. In a process for concentrating dilute phosphoricacid in a phosphoric acid process characterized by a plurality ofseparate phosphoric acid cycles, wherein the phosphoric acid isconcentrated in each cycle by a vacuum evaporator, the improvementwherein the phosphoric acid in the phosphoric acid cycle is heated byindirect heat exchange with liquid water under pressure which isconducted in a closed cycle and heated by indirect heat exchange withsulfuric acid within an SO₃ absorber of a sulfuric acid contact process,said sulfuric acid having a temperature of 80°-200° C.
 2. A processaccording to claim 1 wherein said absorber is an end absorber.
 3. Aprocess according to claim 1 wherein said absorber is an interstageabsorber.
 4. A process according to claim 1 wherein the phosphoric acidin a phosphoric acid cycle associated with a first vacuum evaporator isindirectly heated in two heat exchangers which are connected inparallel, one of which is supplied with sulfuric acid from an end SO₃absorber, the other of which is supplied with part of the sulfuric acidfrom the interstage absorber and the phosphoric acid in the phosphoricacid cycle associated with a second evaporator stage is heated by theremaining sulfuric acid from the interstage absorber.